Deposition Mechanisms.

The success of inhalation therapy is not only dependent upon the pharmacology of the drugs being inhaled but also upon the site and extent of deposition in the respiratory tract. Similarly, the toxicity of environmental and industrial particulate matter is affected not only by the nature of the dust but also by the amount and spatial distribution of deposited particles in the lung. Aerosol deposition is primarily governed by the mechanisms of inertial impaction, gravitational sedimentation, Brownian diffusion, and, to a lesser extent, by turbulence, electrostatic precipitation, and interception. The relative contribution of these different mechanisms is a function of the physical characteristics of the particles, the lung structure, and the flow patterns. Large particles (>5 μm) tend to deposit mainly in the upper and large airways, limiting the amount of aerosols that can be delivered to the lung. Small particles (<2 μm) deposit mainly in the alveolar region, whereas particles in the size range 2-5 μm deposit preferentially in the central and small airways.